The major problem in chemically characterizing and determining the tumor-specificity and clinical applications of carcinoembryonic antigen (CEA) has been the use of polyclonal antiserum reagents, since CEA has been shown to contain a number of antigenic sites which could only be defined and characterized to some extent by cumbersome and mostly inexact and incomplete antibody adsorption techniques. Thus, at least twelve cross-reacting antigens have been described by the use of antiserum prepared against what was claimed to be "purified CEA". However, such "purified CEA" still contained variable quantities of these cross-reactive antigen determinants. For example, anti-blood group A antiserum is able to bind CEA as well as glycopeptide fragments of CEA.
The CEA molecule may also bear determinants which are cross-reactive with the fetal sulfoglyco-protein antigen often found in the gastric juice of gastric cancer patients. Non-specific crossreacting antigen (NCA) is one of the most abundant CEA-crossreactive determinants, occurring in high quantities in digestive tumors, normal colon, normal lung, and normal spleen, as well as in certain white blood cells, e.g., granulocytes, including malignant ones.
In addition to these known crossreactive determinants of CEA, it is likely that others exist which could interfere with the specificity of assays used to detect CEA, e.g., in vitro immunoassays, in vivo radioimmunodetection and therapy and in vitro immunohistochemical detection. Therefore, the characterization and isolation of such crossreactive substances is of interest. One useful outgrowth is the production of antisera having precise immunoreactivity, e.g., specificity to an epitope found on only one antigen in a class of closely related and cross-reactive antigens, or specificity to an epitope common to two or more of such antigens. These antisera are usful in numerous clinical and laboratory settings. Epitope is defined as a single determinant of an antigen or immunogen which influences its specificity (Dictionary of Scentific and Technical Terms, McGraw-Hill, New York, 1976).
In the past, attempts to produce monospecific anti-CEA antisera employed exhaustive adsorption with normal tissue extracts or other materials but these methods were cumbersome, difficult to control, imprecise and still beset with the problem of having crossreactive immonoglobulins present in the polyclonal antiserum.
Recently, attempts have been made to overcome the foregoing problems by the use of monoclonal antibodies. However, even the few prior attempts to produce monoclonal antibodies against CEA have failed to elucidate the different species of anti-CEA monoclonal antibodies based upon a categorization of the epitopes of CEA to which they are reactive. Thus, the extent of true CEA specificity must still await a precise characterization of the epitopes found in the family of CEA antigens and/or the elucidation of epitopes found on antigens which are cross-reactive with CEA. CEA-family antigens are defined as a group of glycoprotein or protein substances that have certain physiochemical and immunological properties in common with CEA.
It is known to use both antigens and specific antibodies in immunoassays. However, such assays have been limited to the use of antigens and antibodies which often have had ill-defined epitope specificities. Accordingly, certain applications forming a part of the present invention would not have been possible heretofore. Similar limitations in the fields of immunohistochemistry, in vivo imaging and tumor therapy are overcome by virtue of the present invention.